As the mud is circulated through the drill string and returned through the surrounding annulus, the drill bit may be cooled and the drilled cuttings may be circulated to the surface. Solid particles are often added to the mud to obtain a suspension with specified properties that can facilitate the drilling process. For example, weighting agents, such as barite particles, are added to increase the fluid density so that the mud can provide enough hydrostatic pressure to prevent formation fluid or gas from entering the wellbore and/or causing a well kick. However, in some instances, added solid particles and borehole cuttings may settle out from the mud either at the bottom of the borehole or on the bottom-side of an inclined wellbore. This problem is known as “sag” and may lead to unstable fluid rheological behavior. If solid particles settle downward, the drilling mud becomes density stratified. The created pressure imbalance may further accelerate the separation process and may lead to stuck drilling pipe, loss of circulation, and/or misdirection of the drilling path.
Conventionally, the occurrence of a mud sag is detected by comparing the mud-out weight (i.e., as the mud leaves the wellbore) and the mud-in weight (i.e., as the mud enters the wellbore) measurements. However, the long time-delay between these two measurements significantly affects the method's accuracy and may lead to delay in activating sag mitigation procedures. Furthermore, uncertainties associated with the sensors can also lead to inaccurate predictions (or uncertainties) of the mud's sag tendency.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that may be encompassed together with one or more of the given embodiments in the scope of the appended claims.